The present invention is directed to methods and apparatus for dissipating heat in a voice coil of a loudspeaker, which improves heat transfer from the voice coil to a heatsink.
Loudspeakers (commonly called “speakers”) are designed for the reproduction of audio signals having a frequency range of approximately 20 Hz to 20 kHz and a pressure range of approximately 10−5 to 50 pascals, or 10−9 to 7×10−3 lbf/in.2.
A loudspeaker system normally includes one or more drivers (a transducer mechanism without a structural radiation enclosure), a crossover network (ensuring that a received electrical drive signal is within an optimum frequency range), and an enclosure. Loudspeakers are used in many different consumer products, such as home and automobile stereos, television and radio receivers, electronic musical instruments, toys, etc. Loudspeakers are also used in any number of professional applications, such as in broadcast stations, recording studios, concert halls, etc.
Loudspeakers may be classified in accordance with several factors, including type of radiation, type of driving element, reproduction range, and diaphragm shape. The type of radiation may include direct radiation and horn-loaded radiation. The driving element may be a magnetic element, an electrostatic element, a piezoelectric element, an ionophone element, or an air-jet element. Magnetic driving elements include dynamic (moving-coil, ribbon, etc.), moving-armature, and magnetostrictive technologies. Reproduction ranges include low frequency (woofer and subwoofer) ranges, mid-frequency (midrange and squawker) ranges, high-frequency (tweeter and super-tweeter) ranges, and full-ranges. Diaphragm shapes include cone (e.g., straight, parabolic, flared, etc.), dome, and flat shapes.
A commonly used loudspeaker classification is the dynamic (moving-coil) direct-radiator loudspeaker. In this type of loudspeaker, a permanent magnet produces a high flux density in a narrow air gap in which a moving voice coil is located. The interaction of the flux of the permanent magnet and an alternating current flowing within the voice coil produces a force that moves a diaphragm to achieve a piston action. The movement of the diaphragm causes corresponding acoustic sound waves, which are preferably linearly related to the electrical driving signal in order to produce high fidelity sound. Further details concerning conventional loudspeaker technology may be found in McGraw-Hill, Encyclopedia of Electronics and Computers, pp. 512–518 (2nd ed., 1988).
A significant disadvantage associated with the dynamic (moving-coil) direct-radiator loudspeaker is that it has a relatively low radiation efficiency, i.e., a ratio of sound output power to electrical input power. Indeed, the radiation efficiency of this type of loudspeaker is on the order of 0.5 to 4 percent. This inefficiency generally results in a majority of the electrical input power being converted into heat.
The voice coil of the loudspeaker is the primary heat generating element. Conventional voice coil assemblies include a helical coil of electrical/magnet wire supported by a bobbin. The helical coil may be formed of a single layer or multiple layers of wire. The electrical/magnet wire may take on various shapes, such as IE, round, flat, etc. The bobbin typically consists of a single layer or multiple layers of sheet-like materials, for example, polyimide, aluminum, aromatic fiber, etc. The bobbin is shaped into a desired geometry around which the voice coil is wound. The bobbin supports the voice coil by way of adhesion between the voice coil and the bobbin. Such adhesion may be made to the inside, middle, outside, or a combination inside/outside of the voice coil. As the bobbin is typically used to provide a mechanical connection between the voice coil and the diaphragm (or speaker cone), a relatively high stiffness is desirable. In some instances, multiple layers of material are employed to increase the stiffness of the bobbin. Such layers may be placed in any number of locations along the bobbin to achieve such stiffness.
It is desirable that the bobbin exhibit stable thermal characteristics, particularly because the voice coil produces a significant amount of heat and operates at elevated temperatures. Conventional high-power loudspeakers may employ high-temperature materials in forming the bobbin such that it remains relatively stiff at elevated temperatures. Such materials include high glass transition point materials, i.e., TG and the like. Unfortunately, these high-temperature materials exhibit extremely poor thermal conductivity, which results in a thermal insulation layer between the voice coil and any fluids and/or structures proximate to the bobbin. For example, air, ferrofluids, etc. may occupy volumes within and/or around the bobbin; however, owing to the thermal insulation characteristics of the high-temperature materials utilized to produce the bobbin, relatively poor thermal conductivity is exhibited between the voice coil and such fluids. This disadvantageously increases thermal time constants between the voice coil and any nearby heat wicks (and/or other heatsink structures), and results in the elevation of voice coil temperatures.
Attempts at solving the above-described thermal management issue have been made, including forced air flow, metallic bobbin materials, impregnated bobbin materials, and inside/outside coil assemblies (e.g., a bobbin disposed between two voice coils). Each of these attempts were unsatisfactory. Forced air flow techniques require through-holes in the assembly or increasing the area around the voice coil to permit such air flow. These techniques, however, reduce the magnetic field and degrades performance. Although metallic bobbins exhibit good thermal conductivity, they cause back electro-motive force (BEMF), which further reduces the efficiency of the loudspeaker. Impregnated bobbin materials exhibit only marginal improvements in thermal conductivity, while exhibiting poor bonding strength and in some cases, BEMF. In inside/outside voice coil assemblies, the heat buildup between the voice coil and the bobbin (the bond line) is increased by a factor of two and the bond line exhibits poor thermal conductivity as compared with a single (inside or outside) design. This is so because the bond line is subjected to heat from both sides and any heat transfer out of one of the voice coils must traverse a heat source (the opposite voice coil) to reach ambient fluids.
Accordingly, there are needs in the art for new methods and apparatus for dissipating heat in a voice coil of a loudspeaker, which enjoy relatively high bobbin stiffness, bobbin thermal stability, and low thermal time constants between the voice coil and adjacent heat wicks.